1. Field of the Invention
The present relates to the field of non-volatile integrated circuit analog signal recording and playback wherein an analog signal is directly stored in and read out from a storage cell.
2. Prior Art
U.S. Pat. No. 4,890,259 discloses a high density integrated circuit analog signal recording and playback system wherein an analog input signal is sampled a plurality of times and then, as additional samples are being taken and temporarily held, a prior set of samples of the analog signal are parallel loaded into a plurality of storage sites or cells, each comprising nonvolatile floating gate memory cells, preferably EEPROM cells. In that system, writing of the groups of samples into the respective storage cells is done by repetitively providing a write pulse followed by a read operation for the respective cells to compare the information stored in each cell with the information held by the respective sample and hold circuit. During the successive write read operations, the write pulse is increased in amplitude, with the write pulse to any cell being stopped or decoupled from the cell when the information read from the cell in the last read operation equaled the value held in the respective sample and hold circuit. To provide time for the successive write read operations, a plurality of sample and hold circuits are provided so that an equal plurality of cells may be loaded or written to at one time. Still, because of practical limitations in the number of sample and hold circuits which may be provided and the limited length of time the integrated circuit sample and hold circuits will accurately hold the same values once taken, the length of time available for writing the sample signals to the storage cells in this parallel load fashion is limited. Thus, because each write read cycle takes a finite amount of time, the number of such cycles which may be completed before the same number of samples has again been taken and must similarly be loaded is limited. This in turn limits the resolution of the stored information which may be achieved by each write pulse while still allowing for properly storing samples which may be at either extreme of the storage range, particularly considering temperature variations, chip to chip processing variations and the like.
U.S. Pat. No. 4,627,027 discloses analog storage and reproducing apparatus utilizing nonvolatile memory elements. The apparatus disclosed therein utilizes a source follower type floating gate storage cell in a device which writes to each cell in a single write operation, as opposed to an iterative write process wherein successive write read operations provide and verify storage of the desired analog signal. In the implementation used in this patent the write circuits are completely separate from the read circuits so that during read, any variation in the characteristics of the load will produce a corresponding variation in the output. The constant current load, if ideal, would not create distortion but in reality any practical realization would create some disturbance. In addition, the different conditions between read the write significantly reduce reproduction quality.